1. Field of the Invention
The invention relates to a multi-input multi-output (MIMO) communication system and codeword selection method of space-time codewords.
2. Description of the Related Art
Due to the need for higher data transmission rate in communication systems, techniques for improving transmission data rate and link quality, such as efficiency coding, modulation and signal processing, are heavily researched. To this end, multi-input multi-output (MIMO) communication systems comprising an array of antennas (i.e. multiple antennas) at both the transmitter and receiver are widely used to promise a significant increase in data rate and link quality without bandwidth expansion and thus capable of meeting the formidable service requirements in the next generation wireless communications. The core scheme of MIMO systems is the space-time coding (STC). The two main functions of STC are the spatial multiplexing (SM) and transmit diversity (TD). By transmitting redundant copies of the signal over different transmit antennas, TD, such as space-time block code (STBC) or space-time trellis code (STTC), can provide the excellent link quality. By transmitting the different data of a signal over different transmit antennas, SM, such as layered space-time code (LSTC) or Bell Labs layered space-time (BLAST) techniques, can acquire the high spectral efficiency.
FIG. 1 shows a conventional system applying a STBC, where two symbols are transmitted over spatial and time domain. FIG. 2 shows another STC technique, combined with Q=2 Alamouti's codes. This technique is referred to as the double space-time transmit diversity (DSTTD). The ST codeword for the DSTTD is
  X  :=      [                                        S                          1              ,              1                                                            -                          S                              1                ,                2                            *                                                                        S                          1              ,              2                                                            S                          1              ,              1                        *                                                            S                          2              ,              1                                                            -                          S                              2                ,                2                            *                                                                        S                          2              ,              2                                                            S                          2              ,              1                                            ]  where the code rate is R=2 and diversity gain is 2. Although the DSTTD is a simple coding scheme capable of achieving the diversity gain and spatial multiplexing gain simultaneously, however, its decoding scheme is more complex than that of STBC. Moreover, the codeword structure is inflexible, resulting in poor performance.
TD can improve signal link quality, but the spectral efficiency is poor. Alternatively, SM provides a high data transmission rate, but poor protection against the channel fading. Thus, a tradeoff between TD and SM is required to achieve the optimum performance.
It is shown that receiver operations should be simple, so as to provide efficient decoding and offer reduced receiver design complexity. In response to the STC at the transmitter side, the receiver is preferable be capable of decoding the coded signal and performing interference cancellation and signal detection for the received signal.
Although ML detection offers the best performance, its computational complexity is higher than other methods. A reasonable solution between the performance and computational complexity is the ordered successive interference cancellation (OSIC) detection that performs the interference cancellation and signal detection for received signal by sorting and using an operation resultant from a previous iteration. For example, if the signals from mobile user 1 and mobile user 2 are both received by the receiver, the signal from mobile user 1 is detected first and interference therein is cancelled to obtain the signal from mobile user 2 based on the OSIC detection. The computational complexity of OSIC detection is lower than that of ML, but achieves the similar performance.
In a time-varying wireless channel, both the transmitter and receiver require an adjustable transmission strategy for data transmission to handle signal failure in an attempt to achieve the best service quality. Thus, adjustable transmission strategies for data transmission in MIMO systems are desirable.